Abstract

During the slug stage of cellular slime mold Dictyostelium discoideum, up
to 105 cells coordinate their movement and migrate as a single organism.
Slugs have a cylindrical shape with tip and tail; their morphological polarity
corresponds to the polarity of migration. A large body of results suggest that
cyclic AMP-mediated cell-cell signaling is the mechanism coordinating
multicellular movement. Waves of cyclic AMP generated at the anterior tip
propagate towards the tail and induce the chemotactic movement of cells
toward the tip. Slugs exhibit highly sensitive environmental reactions:
phototaxis, chemotaxis and thermotaxis. Although many studies have
investigated how Dictyostelium slugs move toward a light source, the
mechanism of phototaxis is still unclear. It has been known that slugs turn
towards the light at the anterior end. In addition, previous research identified
mutations and drug treatments that interfere with phototaxis but the strategy
for analyzing phototaxis has been limited to low resolution both temporarily
and spatially.
In this thesis methods have been developed to analyze phototactic behavior
on two different scales, the slug level and cellular level. The analyses
revealed dynamic features of slug behavior during phototaxis which have
not been previously described. Following light irradiation slugs moved with
approximately 50% higher speed; they showed prominent serpentine
movement of their tip as if they were scanning and correcting migration
direction; they elongated and decreased the diameter of their body; and their
tip remained lifted off the substrate for long periods. The analysis of cell
movement during phototactic turning showed that the cell movement pattern
was unlike any predicted from earlier hypotheses. Some cells in the anterior
zone moved away from the light source across the slug, thus increasing
the volume on the “dark” side (“asymmetric cell accumulation”) and bending
the anterior zone like a lever-arm toward the light source. Furthermore, it was discovered that light irradiation enhances secretion of cyclic AMP from
the slug and that light interferes with cyclic AMP cell-cell signaling during
other multicellular stages as well. A model for phototaxis has been proposed
based on these results. Laterally irradiated light is focused on the distal side
of the slug by a lens effect and locally induces cyclic AMP release. Some
cells accumulate chemotactically on the side away from the light source and
cause a bending of the anterior zone towards the light source. Since cell
movement within the slug is organized by cyclic AMP waves, light induced
cyclic AMP release interferes with the endogenous signaling pattern. The
consequence is an overall change in the shape and the behavior of slug.
The mechanism by which light induces the release of cyclic AMP from slug
cells may involve a histidine kinase phosphorelay pathway, since such a
pathway is known to be functional in Dictyostelium and is used for
environmental responses in many other organisms.